The present invention relates to a multi-piece solid golf ball having rebound characteristics as good as a conventional two-piece solid golf ball when hitting at high head speed, and having better rebound characteristics than the conventional two-piece solid golf ball when hitting at low head speed. That is, the present invention relates a multi-piece solid golf ball having such a structure that golfers who swing a golf club at low head speed at the time of hitting are not at a disadvantage.
Solid golf balls are classified into two-piece golf ball and one-piece golf ball. The two-piece golf ball is mainly used for round play of amateur golfers. The two-piece golf ball has excellent flight distance, but has hard and poor shot feel. The performance of the two-piece golf ball depends on the head speed at the time of hitting, and the two-piece golf ball typically has long flight distance when hitting at high head speed and has short flight distance when hitting at low head speed. However, since it happens often that a golfer who swings a golf club at low head speed at the time of hitting and a golfer who swings a golf club at high head speed at the time of hitting are in a same group at a round play, a golf ball that golfers who swing a golf club at low head speed at the time of hitting are not at a disadvantage, is required.
It has been attempted to improve the defect of the solid golf ball by various means. As a representative example, multi-piece solid golf balls, such as a three-piece solid golf ball obtained by placing an intermediate layer between a core and a cover of the two-piece solid golf ball (as described in, for example, Japanese Patent Kokai Publication Nos. 244174/1992, 142228/1994 and the like), and a four-piece solid golf ball of which the intermediate layer is formed into two-layer structure (as described in, for example, Japanese Patent Kokai Publication Nos. 266959/1997, 179797/1998, 179798/1998 and the like) are proposed.
In Japanese Patent Kokai Publication Nos. 244174/1992 and 142228/1994, a three-piece solid golf ball comprising a core formed from rubber composition, an intermediate layer and a cover formed from thermoplastic resin is described.
In Japanese Patent Kokai Publication No. 266959/1997, a four-piece solid golf ball comprising a three-layer structured core formed from rubber composition and a cover formed from thermoplastic resin is described, and a main object thereof is to improve a flight distance, shot feel and controllability, particularly shot feel and controllability at approach shot when hitting by an iron club.
In Japanese Patent Kokai Publication No. 179797/1998, a four-piece solid golf ball comprising a core formed from rubber composition, an inner intermediate layer formed from thermoplastic resin, an outer intermediate layer formed from rubber composition and a cover formed from thermoplastic resin is described. In Japanese Patent Kokai Publication No. 179798/1998, a four-piece solid golf ball comprising a core and an inner intermediate layer formed from rubber composition, an outer intermediate layer and a cover formed from thermoplastic resin is described. The two four-piece solid golf balls are designed to increase the launch angle and flight distance.
The multi-piece solid golf ball, when compared with the two-piece golf ball, has better shot feel while maintaining excellent flight performance, because the multi-piece golf ball can accomplish a various of hardness distribution. However, it is not considered that the rebound characteristics depend on the head speed at the time of hitting.
Therefore, the present inventors have proposed a golf ball, of which the rebound characteristics do not depend on the head speed at the time of hitting because of a difference in deformation behavior when hit at high head speed and low head speed by making the core a three-layer structure and adjusting the hardness of the middle layer to low (Japanese Patent Application Nos. 14422/1999, 14434/1999, 263628/1999 and the like). However, there was problem in the golf ball that sufficient rebound characteristics were obtained and sufficient flight performance was not obtained when hit at low head speed.
In addition, a method of adding various organic sulfide compounds to a conventional core has been proposed in order to improve the rebound characteristics and extend the flight distance (Japanese Patent Kokai Publication Nos. 228867/1984, 122273/1997, 80503/1998, Japanese Patent Nos. 2669051, 2778229, etc.).
A solid golf ball formed from a rubber composition comprising 4,4xe2x80x2-dithio-bis-dimorpholine and/or derivatives thereof is described in Japanese Patent Kokai Publication No. 228867/1984. A solid golf ball consisting of a core, inside cover and outside cover that the core is formed from a rubber composition comprising 0.05 to 5.0 parts by weight of organic sulfide compound based on 100 parts by weight of base rubber is described in Japanese Patent Kokai Publication No. 122273/1997. A solid golf ball consisting of a core that is formed from a rubber composition comprising bis(2,5-dichlorophenyl) disulfide and a cover is described in Japanese Patent Kokai Publication No. 80503/1998. A one-piece golf ball formed from a rubber composition comprising an organic sulfide compound selected from thiophenols, thiocarboyxlic acids and metal salts thereof, and a multi-layer golf ball that the core is formed from the rubber composition, and is covered by a cover directly or through a middle layer formed on the core are described in Japanese Patent No. 2669051. A multi-layer solid golf ball that the core formed from a rubber composition comprising one or more organic sulfide compounds selected from thiophenols, thiocarboxylic acids, sulfides, zinc salt of thiophenols and zinc salt of thiocarboxylic acids, is covered by a cover directly or through a middle layer formed on the core, is described in Japanese Patent No. 2778229.
In these golf balls, the rebound characteristics when hit at low head speed are improved as a result of using an organic sulfide compound in the core, but the rebound characteristics when hit at high head speed are also improved, and therefore the rebound characteristics depend on the head speed at the time of hitting. In addition, the organic sulfide compound is uniformly dispersed in the core, in the conventional methods of using the organic sulfide compound. Thus the organic sulfide compound is present in a portion of the core where it is not necessary and the organic sulfide compound is not present in sufficient amount in a portion of the core where it is necessary.
A main object of the present invention is to provide a multi-piece solid golf ball having excellent rebound characteristics when hitting at low head speed because the rebound characteristics do not depend on the head speed at the time of hitting.
According to the present invention, the object described above has been accomplished by providing a multi-layer solid golf ball comprising a core consisting of an inner core, a middle core and an outer core, and a cover formed on the core, and adjusting a JIS-C hardness of the inner core and outer core, a hardness difference between the inner core and middle core, a hardness difference between the outer core and middle core and the amount of organic sulfide compound in the inner core, middle core and outer core to a specified range. The present invention can provide a multi-piece solid golf ball having excellent rebound characteristics when hitting at low head speed because the rebound characteristics do not depend on the head speed at the time of hitting.
This object as well as other objects and advantages of the present invention will become apparent to those skilled in the art from the following description with reference to the accompanying drawings.